This invention relates generally to land-based, i.e., industrial gas turbines and, more particularly, to a gas turbine bucket tip clearance control system including a flow circuit within a turbine outer shroud that controls a temperature of the outer shroud via a thermal medium.
Hot gas path components in gas turbines typically employ air convection and air film techniques for cooling surfaces exposed to high temperatures. High pressure air is conventionally bled from the compressor, and the energy of compressing the air is lost after the air is used for cooling. In current heavy duty gas turbines for electric power generation applications, the stationary hot gas path turbine components are attached directly to massive turbine housing structures, and the shrouds are susceptible to bucket tip clearance rubs as the turbine casing thermally distorts. That is, the thermal growth of the turbine casing during steady state and transient operations is not actively controlled, and bucket tip clearance is therefore subject to the thermal characteristics of the turbine. Bucket tip clearance in these heavy duty industrial gas turbines is typically determined by a maximum closure between the shrouds and the bucket tips (which usually occurs during a transient) and all tolerances and unknowns associated with steady state operation of the rotor and stator.
In some turbine designs, the stage 1 bucket is unshrouded because of complex aerodynamic loading and the stress carrying capability of the bucket. That is, the stage 1 bucket tip has no sealing mechanisms to prevent hot gas from flowing over the bucket tip. It is desirable to maintain a minimum clearance between the bucket tip and the turbine inner shroud so that an amount of hot gas flow that bypasses the turbine (and therefore is not expanded for work) is minimized.
In an exemplary embodiment of the invention, a bucket tip clearance control system forms part of a turbomachinery apparatus including a casing, an outer shroud in a slip fit configuration with the casing, and an inner shroud coupled to the outer shroud. The tip clearance control system includes a flow circuit for a thermal medium, wherein the flow circuit defines a flow path within the outer shroud. A thermal medium source is provided in fluid communication with the flow circuit and delivers the thermal medium to the flow circuit in a predefined condition according to operating parameters of the turbomachinery apparatus, such as steady state operation and transient state operation. The temperature of the outer shroud is controlled according to the predefined temperature conditioning of the thermal medium.
Preferably, the outer shroud of the turbomachinery apparatus includes an upper half secured to a lower half at the horizontal engine split line. In this context, the flow circuit may include at least two cavities in the outer shroud, one of the cavities being disposed adjacent the split line. The flow circuit may include a first flow path within the upper half of the outer shroud and a second flow path within the lower half of the outer shroud. In this context, the flow circuit preferably includes at least two cavities in each of the first flow path and the second flow path, one of the cavities in each of the first and second flow paths being disposed adjacent the split line. In one arrangement, the flow circuit includes four cavities in the outer shroud. These cavities preferably communicate via at least one hole from cavity to cavity or via an array of metering holes from one cavity to another cavity.